Considering the thermal performance of a building, the roof is a significant weak point. During the day, most of the heat provided by solar radiation penetrates through the roof into the building. On hot summer days, the living space underneath the roof often overheats which leads to a substantial decrease in thermal comfort. Overnight, a substantial amount of the heat stored inside the building during the day is lost through the roof. On cold winter days, this significant heat loss through the roof has a large influence on the heating demands of the building.
The problem can be solved by the application of phase change material in roof structures. Phase change material is a highly-productive thermal storage medium which possesses the ability to change its physical state within a certain temperature range. When the melting temperature is obtained during a heating process, the phase change from the solid to the liquid state occurs. During this melting process, the phase change material absorbs and stores a large amount of latent heat. The temperature of the phase change material remains nearly constant during the entire process. When the phase change is complete, a continuing heating process leads to a further temperature increase and the absorption of a much smaller amount of sensible heat. In a cooling process of the phase change material, the stored latent heat is released into the environment in a certain temperature range, and a reverse phase change from the liquid to the solid state takes place. During this crystallization process, the temperature of the phase change material also remains constant. The high heat transfer during the melting process and the crystallization process, both without any temperature change, is responsible for the phase change material's appeal as a source of heat storage.
In order to contrast the amount of latent heat absorbed by a phase change material during the actual phase change with the amount of sensible heat absorbed in an ordinary heating process, the ice-water phase change process will be used. When ice melts, it absorbs an amount of latent heat of about 335 J/g. When the water is further heated, it absorbs a sensible heat of only 4 J/g while its temperature rises by one degree C. Thus, water needs to be heated as long as its temperature rises from 1° C. to about 84° C. in order to absorb the same amount of heat which is absorbed during the melting process of ice.
In addition to ice (water), more than 500 natural and synthetic phase change materials are known. These materials differ from one another in their phase change temperature ranges and their latent heat storage capacities.
Currently, crystalline alkyl hydrocarbon phase change materials having different chain lengths are used in textile applications of phase change materials and more specifically in garment applications. Characteristics of these phase change materials are summarized in Table 1.
TABLE 1Crystalline alkyl hydrocarbonsLatentCrystallineMeltingCrystallizationheat storagealkyltemperature,temperature,capacity,hydrocarbonsFormula° C.° C.J/gHeneicosaneC21H4440.535.9213EicosaneC20H4236.130.6247NonadecaneC19H4032.126.4222OctadecaneC18H3828.225.4244HeptadecaneC17H3621.716.5213HexadecaneC16H3416.712.2237
The crystalline alkyl hydrocarbons are either used in technical grades with a purity of approximately 95%; or they are blended with one another in order to cover specific phase change temperature ranges. The crystalline alkyl hydrocarbons are nontoxic, non-corrosive, and non-hygroscopic. The thermal behavior of these phase change materials remains stable under permanent use. Crystalline alkyl hydrocarbons are byproducts of petroleum refining and, therefore, inexpensive. A disadvantage of crystalline alkyl hydrocarbons is their low resistance against ignition.
Salt hydrates are alloys of inorganic salts and water. The most attractive properties of salt hydrates are the comparatively high latent heat storage capacities, the high thermal conductivities and the small volume change during melting. They are mostly non-combustible which makes them specifically attractive for building applications. Salt hydrates often show an incongruent melting behavior as a result of a lack in reversible melting and freezing making them unsuitable for permanent use. Salt hydrates with reversible melting and freezing characteristics are summarized in Table 2.
TABLE 2Salt hydratesLatentMeltingheat storagetemperature,capacity,Salt hydrates° C.J/gCalcium cloride hexahydrate29.4170Lithium nitrate trihydrate29.9296Sodium hydrogen phosphate dodecahydrate36.0280Sodium thiosulfate pentahydrate49.0200Lithium acetate dihydrate56.0270Magnesium cloride tetrahydrate58.0180
There are several thermal effects which can be obtained by a phase change material application in a certain product, such as:                A cooling effect, caused by latent heat absorption of the phase change material.        A heating effect, caused by latent heat release of the phase change material.        A thermo-regulating effect, resulting from either latent heat absorption or latent heat release of the phase change material.        
The efficiency of each of these effects is determined by the latent heat storage capacity of the phase change material, the phase change temperature range and the structure of the carrier system.
The total latent heat storage capacity of the phase change material in a product depends on the phase change material's specific latent heat storage capacity and its quantity. In order to obtain a successful phase change material application, the phase change temperature range and the application temperature range need to correspond.
Phase change materials have been suggested for the use in building constructions. For instance, U.S. Pat. No. 4,587,279 describes the direct addition of phase change material into the wet mix stage of concrete. However, this approach has lead to a substantial reduction of the concrete's mechanical properties.
U.S. Pat. No. 6,230,444 reports a building conditioning technique where phase change material is used in floors and ceilings of rooms in order to minimize the floor to ceiling temperature gradient of the room. However, in this case only the thermal comfort of the specific room equipped with the phase change material can be improved. The thermal comfort of adjacent rooms located in the same building can only be improved if each of them is equipped with phase change material.